This study is concerned with the ways certain physiological and behavioral signs of arousal or irritability (e.g., heart rate variability, vagal tone, colic, sleeplessness, crying) in the first five months of life are related to measures of the child's temperament, emotional expressiveness, and physiology at later ages. By observing patterns of infant-mother interaction both at home and in laboratory settings, we further expect to determine whether individual differences in maternal behavior interact with early tendencies (as indexed by the signs listed above) in determining patterns of psycho-physiological functioning, emotional expressiveness, attachment behavior, and behavioral inhibition in toddlerhood.